1. Field of the Invention
The present invention relates generally to improvements in radio positioning systems and their methods of operation and in particular to use in non-synchronised transmitter networks without the need for additional monitoring receivers.
2. Description of the Related Art
U.S. Pat. No. 5,045,861, the contents of which are hereby incorporated by reference, describes a radio navigation and tracking system which makes use of independent radio transmitters set up for other purposes. The signals from each transmitter, taken individually, are received by two receiving stations, one at a fixed and known location, and the other mounted on the mobile object whose position is to be determined. A representation of the signals received at one receiving station is sent via a link to a processor at the other receiving station, where the received signals are compared to find their phase differences or time delays. Three such measurements, made on three widely spaced independent transmitters, are sufficient to determine the position of the mobile receiver in two dimensions, i.e. its position on the ground. The phase or time offset between the master oscillators in the two receivers is also determined.
xe2x80x9cCURSORxe2x80x9d, as the system described in U.S. Pat. No. 5,045,861 is known, is a radio positioning system which can use the signals radiated by existing non-synchronised radio transmitters to locate the position of a portable receiver. Unlike some other systems which use the temporal coherence properties of networks of purpose-built synchronised transmitters, CURSOR makes use of the spatial coherence of the signals transmitted by single transmitters. In a further development (see U.S. Pat. No. 6,094,168 and U.S. Ser. No. 09/529,914), the technology has been applied to find the position of a mobile phone handset in a GSM or other digital telephone system, and these are examples of an xe2x80x98Enhanced Observed Time Differencexe2x80x99 (E-OTD) method using the down-link signals radiated by the network of Base Transceiver Stations (BTS) of the telephone system.
In the digital mobile telephone application described in U.S. Pat. No. 6,094,168, the contents of which are hereby incorporated by reference, the signals from each BTS within range of the handset are received both by the handset itself and by a fixed nearby receiver, the Location Measurement Unit (LMU), whose position is accurately known. Representations of the received signals are passed to a Mobile Location Centre (MLC) where they are compared in order to find the time difference between them. In FIG. 1 we show the geometry of a standard two-dimensional system. The origin of Cartesian co-ordinates x and y is centred on the LMU positioned at O. The orientation of the axes is immaterial, but may conveniently be set so that the y axis lies along the north-south local map grid. The handset, R, is at vector position r with respect to the LMU position O. A BTS, A, is shown at vector position a.
Consider first the signals from BTS A. The time difference, xcex94ta, measured between the signals received at R and O is given by
xcex94ta=(|rxe2x88x92a|xe2x88x92|a|)/"ugr"+xcex5,
where "ugr" is the speed of the radio waves, xcex5 is the clock time offset between the clocks in the receivers at R and O, and the vertical bars each side of a vector quantity denote that it is the magnitude of the vector which is used in the equation. The value of xcex5 represents the synchronisation error between the measurements made by the two receivers. Similarly, may written for two other BTSs (B and C) at vector positions b and c (not shown):
xcex94tb=(|rxe2x88x92b|xe2x88x92|b|)/"ugr"+xcex5,
xcex94tc=(|rxe2x88x92c|xe2x88x92|c|)/"ugr"+xcex5,xe2x80x83xe2x80x83(1)
The values of xcex94ta, xcex94tb, xcex94tc, are measured by the methods disclosed in U.S. Pat. No. 6,094,168 and the values of a, b, c, and "ugr" are known. Hence the equations (1) can be solved to find the position of the handset, r, together with the value of xcex5.
In U.S. Ser. No. 09/529,914, the contents of which are hereby incorporated by reference, it is described how time offsets between received signals and the receiver""s local clock can be measured using locally-created templates in a GSM telephone system as follows. Suppose that the handset has recorded a short burst of the GSM signals from BTS A. Contained within that recording is the framing structure, synchronisation bursts and other xe2x80x98givenxe2x80x99 data (or predetermined values) which are a constant feature of those transmissions. The processor within the handset can create a matching template, based on the known structure of the network signals. Received signals can then be matched by the locally-generated template. When the template finds a match, the correlation peak at the position of best match corresponds to the time offset between the received signals and the local clock inside the handset. For the signals radiated by BTS A this measured time offset, xcex94ta1, is given by
xcex94ta1=(|rxe2x88x92a|)/"ugr"+xcex1a+xcex51,
where xcex1a is the time offset of the BTS transmissions and xcex51 is the time offset of the handset""s internal clock, both relative to an imaginary universal xe2x80x98absolutexe2x80x99 clock. The signals from BTSs B and C may also be measured in the same way, giving
xcex94tb1=(|rxe2x88x92b|)/"ugr"+xcex1b+xcex51,
and
xcex94tc1=(|rxe2x88x92c|)/"ugr"+xcex1c+xcex51,xe2x80x83xe2x80x83(2)
The same measurements can also be made by the LMU, giving
xcex94ta2=(|a|)/"ugr"+xcex1a+xcex52,
xcex94tb2=(|b|)/"ugr"+xcex1b+xcex52,
and
xe2x80x83xcex94tc2=(|c|)/"ugr"+xcex1c+xcex52,xe2x80x83xe2x80x83(3)
where xcex52 is the time offset of the LMU""s internal clock relative to the same imaginary universal absolute clock. Subtracting equations 3 from equations 2 gives
xcex94ta=xcex94ta1xe2x88x92xcex94ta2=(|rxe2x88x92a|xe2x88x92|a|)/"ugr"+xcex5,
xcex94tb=xcex94tb1xe2x88x92xcex94tb2=(|rxe2x88x92b|xe2x88x92|b|)/"ugr"+xcex5,
and
xcex94tc=xcex94tc1xe2x88x92xcex94tc2=(|rxe2x88x92c|xe2x88x92|c|)/"ugr"+xcex5,xe2x80x83xe2x80x83(4)
where xcex5=xcex51xe2x88x92xcex52. It will be noted that equations 4 are just like equations 1, and can be solved in the same way to find the position of the handset, r, and the value of xcex5.
The methods described above measure time offsets. However, it is sometimes useful to measure phase offsets, frequency offsets, or derivatives of frequency offsets as described later.
It will be apparent that the CURSOR method, in common with all other methods which use the signals from non-synchronised transmitters, requires a network of LMUs to be set up within the coverage area of the telephone system. These units act as reference points at which the unsynchronised signals radiated by the BTSs are measured for comparison with the same signals received by a handset. In another patent application (U.S. Ser. No. 09/830,447) filed simultaneously herewith, we show how the entire network of BTS can be covered using just one xe2x80x9cvirtual LMUxe2x80x9d which acts as the service node for all LMU data. The present invention shows how the CURSOR method (or other E-OTD method) can be applied without the need of a network of real LMUs.
According to the first aspect of the invention there is provided a method of determining the position or change in position or state of motion of a receiver or receivers, the position or change in position or state of motion of which is or are not already known, in a network of transmission sources some or all of whose positions are known, the method comprising
(a) at a first time, measuring the relative offsets in time, phase, frequency, or derivatives thereof with respect to each other, or with respect to a reference source, of the signals received by a first receiver from a plurality of the transmission sources;
(b) at a selected second time, measuring the respective offsets of the signals received by the same or a second receiver whose position or state of motion is not already known from the plurality of the transmission sources; and
(c) calculating the position of the receiver or receivers at the first or second times or the change in position of the first receiver between the first and second times or the state of motion of either receiver from the relationship between the first and second sets of relative offsets.
By xe2x80x9cstate of motionxe2x80x9d is meant a function of the movement of the receiver, including its velocity, acceleration, rate of change of acceleration etc.
The relative offsets in time, phase, frequency, or derivatives thereof, with respect to each other or with respect to a reference source, of the signals received by the first or second receivers from a plurality of the transmission sources may be represented by corresponding offsets or differences in the distances between the transmission sources and the first or second receivers.
It should be understood that, if the second time is measured at a second receiver, the first and second times could be the same.
At selected additional times, the respective relative offsets in time, phase, frequency, or derivatives thereof with respect to each other or with respect to a reference source of the signals received by the first receiver, or by the second receiver, or another receiver or receivers whose positions, changes in position, or states of motion is or are not already known, from a plurality of the transmission sources may be measured; and the position, change in position, or state of motion thereof of any or all of the receivers may be determined using the measurements and known positions of the transmission sources.
In the above aspect, it should be understood that any of the selected times could be the same as any other of the selected times.
The invention also includes a system for carrying out this method, which system may include receivers controlled by computers or microprocessors suitably programmed.